1. Field of the Invention
The present invention relates generally to a writing instrument and, more particularly, to a writing instrument which regulates flow air into a storage container that holds liquid to prevent leakage.
2. General Background and State of the Art
Writing instruments are commonly used to deliver liquids such as ink, paint, adhesives, shoe polish, lotion, medicine, perfume, makeup, white out and food. In one type of liquid dispensing utensil, a relatively large volume of liquid is stored in a non-capillary container (or reservoir) where it is allowed to move freely. Pens which incorporate such a container, for example, are generally referred to as xe2x80x9cfree inkxe2x80x9d pens. U.S. Pat. No. 6,095,707 issued to Kaufmann discloses such a pen. That is, the ink in the reservoir is usually in a liquid state, and is free to move about as the writing utensil is moved. Liquid in these utensils is transferred from the container to the delivery end (often referred to as a tip or a nib) via a capillary conveying line (also referred to as a wick). A slight vacuum (underpressure) relative to the atmosphere is maintained within the container which prevents liquid in the conveying line from escaping from the utensil until the tip is brought into contact with the surface onto which liquid is to be dispensed. At this point, the force of attraction of the surface and the capillary force of the space between the surface and portions of the tip which are not in direct contact with the surface will cause the liquid to flow from the tip to the surface. As liquid is dispensed, air enters the container through the largest pore size in the conveying line. The air replaces the liquid so as to maintain the underpressure within the container at a relatively constant level.
To deal with the problem of leakage caused by air expansion within the container, a capillary storage is used to absorb the excess liquid. Specifically, when the air within the container is heated it expands. This causes the underpressure within the container to subside and increases the vapor pressure on the liquid. This forces excess liquid to flow through the conveying line via capillarity action. To handle the excess liquid, some ink pens include an overflow chamber having a capillary storage that will absorb ink. Fountain pens, for example, include a capillary storage in the front section and sometimes under the nib. This storage has a capillarity that is strong enough to prevent leakage when the pen is held in the writing position, but not so strong that it will be filled during a normal writing operation.
Thus, to optimize the performance of the writing instrument, the pore sizes of the conveying line and storage capillaries need to be carefully controlled. In particular, the largest pore size in the conveying line needs to be carefully controlled. If the largest pore size in the conveying line is too large, then the underpressure within the container may not be held at a relatively constant level because too much air may be flowing into the container. With the underpressure subsiding in the container, excess ink would flow through the conveying line and overwhelm the storage capillary, and ultimately would leak through the nib. On the other hand, if the largest pore size in the conveying line is too small, the underpressure within the container would increase because not enough air is entering the container. This would restrict the flow of ink through the conveying line, thereby drying out the nib. Controlling the largest pore size in the conveying line, however, may be difficult. That is, with current manufacturing methods, the largest pore size in one conveying line may vary from one conveying line to another conveying line, such that one writing instrument may provide excess ink while another writing instrument may not provide enough ink.
Therefore, there is still a need for a writing instrument that can more accurately control air inlet to the container such that the underpressure within the container can be more accurately controlled.
Another problem with the writing instrument is that only one end is used to write. That is, when a conveying line has a relatively large diameter, the writing may be relatively thick. So when a user wants to write in a fine line, the user must change the writing instrument in order to do so. Therefore, there is a need for a writing instrument that can provide for both a wide and a fine writing capability.
One feature of the present invention is to provide a capillary hole through a storage container that is holding the liquid so that as the underpressure within the container increases, atmospheric air may enter through the capillary hole to hold the underpressure at a predetermined level. One of the advantages with the present invention is that air enters through the container wall and not through the conveying line. That is, the diameter of the capillary hole formed in the container may be more precisely controlled than trying to control the largest pore size in the conveying line. This means that the performance of the writing instrument may be held to a tighter tolerance because the underpressure in the container may be more accurately controlled.
Alternatively, a second nib may be provided through the capillary hole on the opposite side of the first nib. In this embodiment, the second nib may be used to control the air inlet through the container and, at the same time, be used for writing purposes thereby providing a writing instrument that can produce writing in both a thick and a fine line.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.